The paper extrusion coating industry currently uses polyolefins and poly(ethylene terephthalate) [PET] for food packaging applications. A thin layer of polymer is extrusion coated onto paper and the paperboard is used in various ways, such as being vacuum formed into containers for food and beverage packaging. Polyolefins are utilized in the paper extrusion coating industry, because of their excellent chemical resistance, low resin price and good adhesive characteristics. The good adhesive characteristics of polyolefins allow them to be extrusion coated onto paper in thin layers. Polyolefins are processed using low extruder motor loads and they have good melt stability, which minimizes the edgeweave (lateral variation in the edge of the coating) during extrusion. Due to their low melting transitions, polyolefins do not have the thermal characteristics needed for ovenable and microwaveable applications. In high temperature oven applications (&gt;150.degree. C.), PET is currently the extrusion coating resin of choice. PET offers good chemical resistance and can withstand end use temperatures of up to 425.degree. F. (218.3.degree. C.); however, at present, it does not have good adhesive properties to the paperboard. The lack of good adhesion requires converters to extrude thicker coating layers of polymer onto the paper to obtain acceptable adhesion, thereby adding to the expense. It was surprising to find that the addition of small quantities of a glycol such as diethylene glycol (DEG) had a significant effect on the adhesive characteristics of linear and branched PET extrusion coated onto paper.
German Patent No. 1,110,412 discloses films of high molecular weight terephthalate polyesters based on at least one glycol HO(CH.sub.2).sub.n OH where n=2-10 and 2 to 5 mol % of a polyglycol which may be diethylene glycol. In this application the diethylene glycol was added to reduce the crystallinity so the that the film could be oriented.
U.S. Pat. No. 4,352,925 discloses heat-resistant adhesive copolyesters derived from terephthalic acid, 30-50 mol % of diethylene glycol and at least 50 mol % of ethylene glycol. These copolyesters are suitable for bonding parts of ovenable food containers.
U.S Pat. No. 4,156,774 discloses thermoplastic copolyesters based on 40-50 mol % of terephthalic acid, 0-10 mol % of aliphatic, cycloaliphatic or other aromatic dicarboxylic acids, 15-30 mol % of ethylene glycol and 15-34.5 mol % of diethylene glycol. These copolyesters are suitable as hot melt adhesives, but were not suggested as being useful in extrusion coating applications.
U.S. Pat. No. 5,132,391 discloses polyester adhesive compositions which exhibit increased melt viscosity and improved adhesion. The copolyesters are derived from terephthalic acid, 75-50 mol % of ethylene glycol and 25-50 mol % of diethylene glycol reacted therewith or copolymerized with a phosphite ester and optionally containing a phenolic antioxidant. Also, disclosed are articles coated with the polyester compositions and bonded laminate articles based on the compositions.
U.S. Pat. No. 4,381,356 discloses polypropylene compositions, which contain an inert particulate filler and a copolyester which greatly increases the heat stability of articles produced therefrom. The copolyesters are described as amorphous polyesters derived from terephthalic acid, at least 50 mol % of ethylene glycol and 20-50 mol % of diethylene glycol or 1,4-cyclohexanedimethanol.
The present invention provides copolyesters that may be extrusion coated in very thin layers onto paperboard in conventional extrusion coating processes, the coating having significantly better adhesion to the paper than conventional PET.